PROJECT SUMMARY/ ABSTRACT The long-term goal of the proposed work is to gain a better understanding of how gene expression is regulated during mammalian cell development. This proposal focuses on the study of mRNA, which can be regulated in many ways to provide multiple layers of gene expression control. In order for cells to progress through different stages of development, its mRNAs must undergo extensive stage-specific changes. Defects in mRNA regulation are the direct cause of many human diseases, thus a better understanding of how mRNAs are regulated is essential. Our plan is to isolate mouse cells that are in specific stages of development and perform comprehensive analyses to identify changes in mRNA regulation, understand the functional significance of these changes, and determine how these changes are controlled at the molecular level. To do so, we have developed a new approach to purify male mouse germ cells in each of the major stages of gametogenesis (the process by which sperm are generated). Combining this new cell isolation strategy with powerful genetic, biochemical, deep sequencing, and bioinformatic tools, we will be able to perform comprehensive, global analyses of multiple layers of mRNA-based gene control in wild type mice as well as mice lacking an essential protein necessary for proper RNA regulation. In doing so, we will gain important insights into mechanisms and functions of mRNA regulation in different stages of cell development in unprecedented molecular and cellular detail. As a result, this study will lead to an improved understanding of how gene expression is controlled through mRNA regulation during cell development, and has the potential to provide important new insights into human reproductive biology and the molecular mechanisms controlling fertility.